Structure design and construction requires consideration of multiple factors. The design and fabrication process will generally include considerations of: 1) design requirements, 2) likely failure modes, 3) stress analysis for failure modes identified, 4) material selection and behavior, 5) fabrication, and 6) testing, all within the context of the overall design goals. For example, in order to achieve reduction in weight, increase in strength, and reduction in cost, the engineering design, materials of construction, and methods of fabrication must all be considered. In general, modern fabrication techniques include various additive and subtractive processes, employing a range of materials, including, but not limited to, cellular materials, composite materials, and digital materials.
“Cellular materials” or “cellular solids” refers to the material structure of any living or nonliving matter, typically described as anisotropic and unidirectional or isotropic and having the same properties in all directions. Cellular materials can fill space in two-dimensions as extruded honeycomb or prismatic cells or three-dimensions as space filling polyhedra in various lattice formations. Cellular materials have been mimicked in engineered foam core structures used in construction, aerospace, and medical industries. These man made materials can be designed as highly porous scaffolds or fully dense structures which can be mechanically tuneable for a specific performance. While the science of cellular solids has enabled widespread use of lightweight materials to meet many important engineering needs, such as passive energy absorption, cellular solids are not presently in widespread use for structural applications, perhaps due to a large gap between the strength and stiffness to weight ratios of popular classical solids and the performance of known lightweight cellular materials produced from the same constituent material.
“Composite materials” describes any two materials which are combined together in a single bulk material to obtain the best properties from both materials. Many industries are shifting towards the use of more composite materials because they display the single most significant consideration for any application: low weight compared to strength. The material properties of composites are unlike any material thus far, because they combine the properties of a high modulus and high tensile strength fiber for flexibility and strength, with a low modulus stiff matrix which transfers forces from one fiber to the next, creating essentially a continuous analog bulk material. Fiber-reinforced composite materials have thus enabled construction of structures having large reductions in weight for given strength and stiffness targets, but this reduction comes at the cost of very high design and processing costs and many challenges in producing mechanical interfaces (joints).
Composites are still problematic as the material of choice hindering widespread use for many reasons. First, composites vary in fibers, resins, and weaves from one manufacturer to the next, with strength and weight dependent on layup and direction of weave. Second, composites require an energy intensive process. Highly skilled technicians are never really able to have complete control over the application of pressure and heat to allow for proper curing and even distribution of heat over the entire surface [Dorworth, C. Louis, Gardiner L. Ginger, Mellema M. Greg, “Essentials of Advanced Composite Fabrication and Repair”, 2010]. Third, any flaw detected in a composite skin renders the entire material a complete waste, or makes repair difficult since creating the exact conditions to maintain bond strength is close to impossible to achieve. Fourth, not only is the composite surface designed, but the tooling and moulding for the composite is just as intensive as the final part. In the process of mitigating stress concentration, composite skins are ultimately labor intensive, time intensive, and expensive.
In contrast, digital materials promise scalable methods of producing functional things with reconfigurable sets of discrete and compatible parts. Digital materials are comprised of a small number of types of discrete physical building blocks that may be assembled to form constructions that have a level of versatility and scalability that is analogous to that of digital computation and communication systems. Digital materials have specifically been defined in prior work by Popescu as having three main properties at the highest level of description: a finite set of components or discrete parts, a finite set of discretized joints of all components in a digital material, and complete control of assembly and placement of discrete interlocking components [Popescu, G., Gershenfeld, N. and Marhale, T., “Digital Materials For Digital Printing”, International Conference on Digital Fabrication Technologies, Denver, Colo., September 2006].
A digital material desktop printer, now called the MTM Snap, was the first application constructed entirely out of discrete, snap-fit, reversible digital materials The entire structure for the MTM Snap is made up of a finite set of discrete parts, with built-in flexural connections and slots that are all milled as one CAD file on any CNC shopbot machine. The parts for the machine are made of high density polyethylene, which as a material demonstrates great potential to create robust and stiff flexural connections, although it can be made out of many other suitable materials. The entire machine can be fabricated within a day, with additional motors and tool heads installed depending on the fabrication method desired. These digital material printers can print or mill their own parts, in order to replicate and build more machines like themselves. Current work at MIT's Center for Bits and Atoms is taking the digital material printer to the next level, by incorporating a pick and place mechanism, called a digital material assembler, which is a machine that picks and places each newly fabricated piece to create the final form. The possible properties of digital materials are myriad, and they can be designed out of any material using existing fabrication technologies and tools in order to build cellular structures for any application. Digital materials, as compared to analog materials, are completely reversible, eliminating waste by allowing individual parts to be reused and recycled at any point in the product lifecycle, no matter how large the assembly.
Several additive methods that use discrete components to create an analog material as a final product are known in the art. Selective laser sintering (SLS) uses high power lasers to fuse powders such as glass, metal or thermoplastics, creating forms that are irreversible. The powders are not analog, but are initially formless particles that are discrete and separate. Upon fusing a particle to another, a new analog material is created that is continuous and attached to adjacent particles to form the larger object. Another such additive method is fused deposition modeling (FDM). FDM takes a coil of thermoplastic or metal wire and deposits material from an extruder by heating and melting the material. Stereolithography (SLA) is similar to SLS, but instead of using powder, uses a vat of liquid with a high power laser to create the part in cured layers [Bourell, D. L., Leu, M. C. & Rosen, D. W. (Eds.) Roadmap for Additive Manufacturing: Identifying the Future of Freeform Processing, Austin, Tex., The University of Texas at Austin Laboratory for Free-form Fabrication, 2009]. Electron beam melting (EBM) is another additive process prevalent in the aerospace industry, and uses an electron beam to melt metals such as titanium in powder form. Similar to previous processes, each part is built one layer at a time, solidified and a subsequent layer is built. Current additive manufacturing technologies may utilize the same materials used in manufacturing processes, but the final products rarely behave per material specification, always depend on the machine for surface resolution, and any error in the part generates wasted material.
Conversely, subtractive manufacturing processes take solid blocks or sheets of material and machine out material by drilling or milling from the existing material to create the final part. The initial material is analog in nature, but often these discrete parts are combined within larger assemblies using irreversible joining and bonding methods which again, render the assemblies irreversible, with surface resolution depending on the machine tools used, and any error in the part means waste of the entire assembly of materials. For any given additive or subtractive process, representation of the initial model and translation from initial design to final product requires greater integration than the tools currently offer. Digital fabrication is currently only great at rapid prototyping and not for creating reliable, robust functional structural components yet.
Many things in nature exhibit a sandwich structure, comprising a skin with an exterior and interior contents that support the skin. Depending on the mass of the matter or structure, the moment of inertia is an important property which can be derived from the cross section of any structure, dictating whether the material is strong and able to resist bending and buckling forces. The moment of inertia increases as the cross section of any given shape such as a tube also increases, provided that the radius divided by the tube thickness ratio also increases. There are many similarities between a digital material and a cellular material. By preserving these concepts and translating them to other materials such as metals, plastics or composites, structural skins can be created with the best resistance to bending and buckling, with specific tuneable properties.
Structural skins, such as those used in pressure vessels, are envisioned as a primary application for digital materials. Pressure vessels are leak proof containers for containing matter, whether present as solid, liquid, or gas. In particular, spacecraft missions have traditionally sacrificed fully functional hardware and entire vehicles to achieve mission objectives. Propellant tanks, which are pressure vessels, are typically jettisoned at different stages in a spacecraft mission and left to burn in the atmosphere after one use, creating a substantial amount of waste and redundancy which leads to high operational costs. Spaceflight programs cannot continue to rely on current methods of discarding hardware, since the cost to transport materials from Earth is extremely high. Significant improvements need to be made in recovery and reuse of valuable hardware, to be able to lower costs per mission and increase the number of missions. Strategies therefore need to focus on avoiding complete loss of hardware.
Pressure vessels can have any combination of geometries based on cylinders, ellipsoids or spheres. Many materials, tools and processes can be chosen, each displaying significant advantages for different metrics such as cost, mass, strength, and vessel life. In a pressure vessel, an axis-symmetrical shell cross section with some internal pressure is subject to radial forces which distribute along the circumference of any cross-section considered. Internal forces translate to hoop stress, which act tangentially in plane with the vessel skin. Spherical pressure vessels are stronger than cylindrical pressure vessels, because stress is uniformly distributed at every point, but are more difficult and expensive to manufacture. The mass of a vessel is proportional to the pressure and volume it contains. To determine stresses resulting from loads, the relationship between stress and strain from an applied static or dynamic load needs to be quantified for a given material, pressure, and vessel thickness. A spherical pressure vessel considered for any given circumference along the cross-section contains two types of stresses: radial and hoop [Harvey R. John, “Theory and Design of Pressure Vessels”, Springer, Sep. 19, 1991]. Hoop stresses occur at every point, in a direction tangential to the plane the point lies on. The vessel skin will try to expand or contract, depending on what direction the radial forces are pointing. To understand the force along a given cross-sectional cut of the spherical vessel with a small thickness (in comparison to other dimensions of the vessel), the vessel must achieve equilibrium of forces for the exterior and interior pressure values.
There are several traditional fabrication methods for continuous pressure vessel skins. Most pressure vessels are made of two or more parts that have been previously fabricated as segments of a cylinder or hemisphere, which are then joined to form the base vessel with additional attachment and openings. Traditionally, three types of processes are used for vessel fabrication: welding, forging, and brazing.
The general welding technique for welding two parallel parts with no overlap is called a butt-weld, used in in conjunction with any of the following pressure welding processes: explosive, flash, thermit, induction, continuous drive friction, inertia, resistance, and gas pressure. No matter what the weld process, any one of those mentioned creates a non-precise fabrication process increasing stress factors or potential failure modes in the vessel. There are several potential failure modes during the welding process. The first area where failure can be introduced is with the structure of the metal being welded when compared to the parent metal, which may or may not have a similar structure. The second is that failure can be introduced is during the welding process creating local deformities in the form of porosity, lack of fusion, slag inclusions and shrinkage cracks. The third is due to local stress concentration as a result of misalignment of the two vessel halves, which have been prefabricated as separate segments. The fourth area is with the weld geometry and surface finish, whereby deformities arise from less than perfect welding techniques. The fourth area has to do with repair of welded vessel joints, which often cost more than just using an existing method to remove and re-weld the two halves. Repairing welds is also risky, because one never knows with all the tests whether the weld has produced in some cases a less than desirable result compared to the original surface of the vessel skin.
All methods from casting of metals, multilayer construction, wire wrapped construction or filament wound vessels, the part production and the joints and their associated processes create flaws which affect the entire assembly or piece. Through these local discontinuities, entire structures eventually undergo global failure. The same failure modes for aircraft skins generally apply to pressure vessel skins and for other skin applications. Structural composites for pressure vessel applications will soon replace steel tanks, since composite vessels are demonstrating a 60% weight reduction, less corrosion, require low maintenance, and normally do not need coating for UV protection.
In a filament wound vessel, a permanent or removable mandrel in the shape and size of the final pressure vessel is wound continuously with a filament. The carbon fiber matrix is applied with a wrapping system oriented in the principal direction of stress incurred in the system. Also, if the longitudinal stress is three times the hoop stress, then the mandrel is filament wound three times the thickness of the hoop direction in the longitudinal direction, to maintain the same stress throughout the skin. The advantages of filament wound vessels include high strength to low weight material properties, high corrosion resistance due to material properties of the resin, and lower notch sensitivity since broken filaments do not affect neighboring filaments.
Multi-layer construction of pressure vessels is another common fabrication method, where the tank system contains an inner shell between ¼″-½″ thick, wrapped and welded in order to incur the membrane stresses. It is created as a concentric layered skin system, with attachments and holes drilled in different layers to be able to monitor leakage. The advantages of multi-layer construction include lower costs since each layer can be a different material, the inner compatible with the matter contained and the outer can be a cheaper material used to hold the tank walls in place. Disadvantages include local stress concentration since layers are not attached, and thermal gradients created by heating and cooling of different layers at different rates.